Hackers have succeeded in flashing a fake rocket attack warning during a webcast of a Eurovision song contest semi-final in an incident Israel's public broadcaster blamed on Islamist group Hamas.

KAN's online coverage of Tuesday evening's first semi-final in Tel Aviv was interrupted by what appeared to be satellite imagery of the seaside city superimposed with graphics of a smoke plume suggesting a rocket strike.

It ended with the sound of an air raid siren and a message in English resembling an official Israeli military warning of impending attack.

Regular television coverage was not affected.

"At a certain point, there was a takeover -- apparently by Hamas -- of our digital broadcast," KAN chief Eldad Koblenz said Wednesday.

"I'm happy to say that within a matter of minutes we managed to overcome the attack," he said in an interview with Israeli army radio.

"I think it was Israel's quickest victory over Hamas in history."

The Jewish state won the job of hosting this year's 64th Eurovision contest after Israeli singer Netta Barzilai's victory in Portugal last year.

A brief but bloody uptick in cross-border violence between Israel and Gaza militants two weeks ago brought fears that the contest -- which culminates in Saturday's final with an expected appearance by Madonna -- could be a target.

The two-day Gaza flare-up ended in a tentative truce after Hamas and Islamic Jihad fired more than 690 rockets and mortar rounds into southern Israel, prompting the military to strike dozens of targets in Gaza.

Four Israeli civilians and 25 Palestinians, including at least nine militants, were killed in the exchanges.

There will be a second Eurovision elimination round in Tel Aviv on Thursday.

Those who win through to Saturday's final will join host Israel and the so-called "big five" of Britain, France, Germany, Italy and Spain -- all of whom are entitled to skip the elimination rounds.

Around 200 million people are expected to tune in for Saturday's show.